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Glycoconjugates. Carbohydrates covalently linked to a protein or lipid. Proteo glycans Glyco proteins Glyco lipids. act as informational carrier in: cell-cell recognition, cell-cell adhesion, cell migration, immune response , …. Glycoconjugates: 1. Proteo glycans.

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Carbohydratescovalently linked to a protein or lipid




act as informational carrier in:

cell-cell recognition,

cell-cell adhesion,

cell migration,

immune response,



Glycoconjugates: 1. Proteoglycans

  • Proteoglycans: major components of connective tissues
    • Macromolecules of the cell surface and extracellular matrix
    • Composition
      • A core protein (integral membrane protein or extracellular)
      • Glycosaminoglycans covalently attached to the protein (through Ser)
        • Main fraction by mass
        • Main site of biological activity
    • Example: syndecan core protein with 3 Heparan sulfate and 2 Chondroitin sulfate chains (bind cell surface receptors)

Trisaccharide linker


Proteoglycan aggregates

Associate with collagen in

extracellular matrix of cartilage,

contribute to the development

and tensile strength

of cartilage

  • Proteoglycan aggregates
    • Supramolecular assemblies
    • Many core proteins bound
    • to a single molecule of hyaluronate

Mr > 2x108

e.g. Aggrecan core protein

bound to chondroitin sulfate

& keratan sulfate;

Many copies bound to a single molecule of Hyaluronate

Take up a

volume equivalent

to a bacteria cell

Extracellular meshwork
    • Formed by extracellular proteoglycans interwoven with fibrous matrix proteins (collagen, elastin, fibronectin) and plasma membrane proteins
    • Roles:
      • Anchor cells to extracellular matrix
      • Direct migration of cells in developing tissues
      • Convey information in and out of cells across the plasma membrane


  • Glycoproteins
    • Carbohydrate-proteinconjugates
    • Carbohydrate moieties are smaller (in many cases) & more structurally diverse
    • Linking point:

anomeric C of reducing end N- or O-linked to protein

      • N-linkage: Asn
      • O-linkage: Ser/Thr
    • N-linked oligosaccharides contain a common core
      • 2 N-acetylglucosamine (GlcNAc)
      • 3 mannose (Man)
    • Additional sugars attached to the core in many different ways
      • High mannose type
      • Complex type
        • GlcNAc, galactose, L-fucose
        • and Sialic acid (negatively charged)

High mannose type

Complex type

glycoprotein glycophorin
Glycoprotein - Glycophorin

An erythrocyte plasma membrane protein:

16 glycoxylation sites:

[15 O-glycoxyl linkage (Ser/Thr)

and 1 N-glycoxyl linkage (Asn)]

Rich in sialic acid

biological roles of glycans in proteins
Biological Roles of Glycans in Proteins
  • In early secretory pathway of glycoproteins in ER
    • Promote protein folding
      • Some proteins are completely dependent on glycosylation
      • Some are partially dependent
      • Others are not dependent on glycans
      • Some are glycan-dependent in one cell type but not in another
      • Some glycosylation sites are more important than others
    • Aid in certain sorting events
  • In later secretory pathway of glycoproteins in Golgi
    • Structural features of glycans act as destination labels

(secreted, membrane proteins, lysosomal proteins…)

  • In final mature forms (intra- or extracellular locations)
    • Cell-cell recognition and adhesion
3 glycolipids and lipopolysaccharides
3. Glycolipids and Lipopolysaccharides
  • Glycolipids:
    • e.g.: Gangliosides
      • Membrane lipids of eukaryotic cells
      • Head group is a complex oligosaccharide containing sialic acid in addition to other monosaccharides
      • Oligosaccharides contribute to the different blood groups
  • Lipopolysaccharides
    • Occurs on the outer membrane of gram-negative bacteria

(e.g.: E. coli, Salmonella typhimurium)

    • Prime targets of antibodies
    • Some bacterial polysaccharides are toxic to humans

Bacterial Lipopolysaccharides

Principal determinant

of serotype (immunologic



inherent to






Permeability barrier:

Selective passage of nutrients

and exclusion of harmful substances